Register to reply

Why sound wave travels faster in humid air than in dry air?

by Ahmed Abdullah
Tags: faster, humid, sound, travels, wave
Share this thread:
Ahmed Abdullah
#1
Jun16-07, 03:50 AM
P: 183
A:The answer is humid air is less dense.
Q:But why humid air is less dense?
A:Because water molecules have less mass than the average air molecules do.
But It will happen if and only if the pressure is unchanged. Given that dry and humid air are both under the same pressure then we have smaller density for humid air. But when a place is saturate with water vapour the pressure increases (i.e for the vapour pressure of the water).
I am thinking of a closed container half filled up with water. At first the air over the water surface is dry but as time passes water molecules from the surface escapes and eventually the air becomes saturated. So the density of the air should increase when air is saturated by water vapour (for the volume is unchanged and there is more molecules).
This is the part which I can't understand.
Please help me.
Phys.Org News Partner Physics news on Phys.org
Mapping the optimal route between two quantum states
Spin-based electronics: New material successfully tested
Verifying the future of quantum computing
Ahmed Abdullah
#2
Jun16-07, 05:06 AM
P: 183
I know that humid air is less dense than dry air.
So it turns out that the atmospheric pressure always remains constant. Volume change in such a way to keep the pressure constant.
Right?

Does it not also implies that the atmospheric pressure does not change with temperature?
Ahmed Abdullah
#3
Jun16-07, 05:27 AM
P: 183
Can someone knowledgable enlighten me?

kiblex
#4
Jun16-07, 06:03 AM
P: 2
Why sound wave travels faster in humid air than in dry air?

"Why sound wave travels faster in humid air than in dry air?"

Is this really true? Isnt the speed of sound in relation only with temperature?

You can calculate the speed of sound: acoustic velocity(a)=sqrt(ratio of specific heat(gamma)*gas constant(R)*temperature(T))

If we have a day with humid air and a temperature of 288K and we have another day with dry air but with a temperature on 300K. The speed of sound will be higher the day with dry air and higher temperature? Or am I wrong here?
Astronuc
#5
Jun16-07, 03:23 PM
Admin
Astronuc's Avatar
P: 21,827
The speed of sound changes with the temperature and a little bit with the humidity, but not with the air pressure.
http://www.sengpielaudio.com/calculator-airpressure.htm

Humidity has a small, but measurable effect on sound speed. Sound travels slightly (0.1%-0.6%) faster in humid air.
http://en.wikipedia.org/wiki/Speed_of_sound

Sound Speed in Gases
http://hyperphysics.phy-astr.gsu.edu...d/souspe3.html

The mean molecular mass of air drops slightly with a little moisture since H2O, M=18, is lighter than N2, M=28, O2, M=32, but the amount of moisture is relatively low.

http://hyperphysics.phy-astr.gsu.edu...souspe.html#c4

For properties of air, see this page about 2/3's of the way down -
Illustration of Variation of Air Properties with Temperature (1 Atmosphere Pressure)
http://www.uigi.com/psychrometry.html
Danger
#6
Jun16-07, 03:42 PM
PF Gold
Danger's Avatar
P: 8,964
Astro, you never cease to amaze me with the things that you can pull out of your beard on a moment's notice.
Ahmed Abdullah
#7
Jun17-07, 04:32 AM
P: 183
Quote Quote by Ahmed Abdullah View Post
I know that humid air is less dense than dry air.
So it turns out that the atmospheric pressure always remains constant. Volume change in such a way to keep the pressure constant.
Right?

Does it not also implies that the atmospheric pressure does not change with temperature?
How pressure of our atmosphere changes with varying temperature?
Thnx
Astronuc
#8
Jun17-07, 05:58 AM
Admin
Astronuc's Avatar
P: 21,827
Quote Quote by Ahmed Abdullah View Post
How pressure of our atmosphere changes with varying temperature?
Thnx
The density of an unconfined (unconstrained) gas decreases with temperature, and the pressure is simply the integral of [itex]\rho(T(z))\,g\,dz[/itex] from the point of interest to the top of the atmosphere.


Register to reply

Related Discussions
Why not faster than speed of sound? Engineering Systems & Design 10
Light acts as a wave but travels in a staright line right.. Quantum Physics 4
If sound moves faster through...? General Physics 3
Question...which one travels faster? General Physics 15
Anti gravity travels faster than light. General Physics 23